I do most of my book shopping on Amazon.com and Christianbook.com. These two bookstores (and probably others) are great because the books I want are most often in stock and I can get them in short order. I can order at 2:00 am if I want. No worries. And I do love the fact that I don't have to drive anywhere. (Idaho gas average is still over $4.00.)
I buy enough books in paper form (yes, I'm still buying e-books for my Kindle and am loving it) that it makes sense for me to have the Prime membership. With it, I get "Free" two-day shipping with all of my book orders and most other orders as well (music, DVDs, replacement heads for my electronic toothbrush). Of course, the shipping isn't really free because I have the annual Prime membership fee to pay, but I save far more than the fee over the course of a year. I used to hold a number of items in my cart before buying, just so I wouldn't have to pay as much in shipping charges. Now I don't wait. I see a book I want, I order it then.
Right now Amazon is offering a free trial of their Prime Membership. So if you already do your book buying on-line, you might want to test out Amazon's Prime and see if it will save you money too.